The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is currently the industry standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkylaromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore a transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA framework type, such as SAPO-34.
CHA framework type molecular sieves appear to be particularly suitable catalysts for the OTO reaction since they have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that increasing the silica to alumina molar ratio seems to be a key requirement in the use of CHA framework type aluminosilicates in OTO reactions.
There is therefore significant interest in developing synthesis routes for the production of CHA framework type aluminosilicates having a high silica to alumina ratio, typically in excess of 100:1, and in particular in developing synthesis routes that are simple, inexpensive and readily amenable to large scale commercial production.
Chabazite is a naturally-occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in “Zeolite Molecular Sieves”, by D. W. Breck, published in 1973 by John Wiley & Sons, the disclosure of which is incorporated herein by reference. The three synthetic forms reported by Breck are Zeolite “K-G”, described in J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962). Zeolite K-G has a silica:alumina mole ratio of 2.3:1 to 4.15:1, whereas zeolites D and R have silica:alumina mole ratios of 4.5:1 to 4.9:1 and 3.45:1 to 3.65:1, respectively, making these materials less than optimal as catalysts fro OTO reactions.
U.S. Pat. No. 4,544,538, incorporated herein by reference, describes the synthesis of another synthetic form of chabazite, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantylammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH-medium. According to this patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50, but it is stated that higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. However, attempts to synthesize SSZ-13 in OH− media at silica to alumina molar ratios in excess of 100 have been unsuccessful and have produced ITQ-1 or SSZ-23, depending on the alkali metal cation present. Moreover, increasing the silica to alumina molar ratio of SSZ-13 by dealumination has met little or no success.
U.S. Pat. No. 6,709,644 describes a zeolite that is identified as SSZ-62 and has a CHA framework-type and a crystal size of 0.5 micron or less. SSZ-62 is said to have a silica to alumina molar ratio in excess of 10, such as in excess of 30, but the only synthesis example produces a material with a silica to alumina molar ratio of 22. Synthesis is effected in a hydroxyl medium in the presence of N,N,N-trimethyl-1-adamantammonium cation as the structure directing agent.
In a paper entitled “Templates in the transformation of zeolites to organozeolites. Cubic P conversions”, ACS Symposium Series (1990), 437 (Novel Mater. Heterog. Catal.), 14 to 24, Zones et al. disclose that the treatment of low-silica zeolite P with a RMe3NI directing agent, where R is cyclohexyl, 2- or 3-methylcyclohexyl; 2-exo- or 2-endo-norbornyl, 2-bicyclo[3.2.1]octyl, 9-bicyclo[3.3.1]nonyl, or 1- or 2-adamantyl, leads to the crystallization of high-silica SSZ-13 containing the directing agent. However, the silica/alumina ratios of the SSZ-13 produced are reported to be in the range 11 to 13.
An all silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using N,N,N-trimethyladamantylammonium in hydroxide form as the structure-directing agent, but the synthesis requires the presence of concentrated hydrofluoric acid. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun. 1881 (1998).
More recently, an aluminosilicate with the CHA framework type and having a silica to alumina molar ratio in excess of 100, such as from 150 to 2000, has been synthesized, again in presence of concentrated hydrofluoric acid. See U.S. Patent Application Publication No. 2003/0176751, published Sep. 18, 2003 and incorporated herein by reference. The directing agent employed is selected from N-alkyl-3-quinuclidinol cations, N,N,N-tri-alkyl-1-adamantammonium cations and N,N,N-trialkyl-exoaminonorbornane cations.
Synthesis processes that involve working with corrosive materials, such as concentrated hydrofluoric acid, present significant practical problems especially on a commercial scale. Moreover, current methods of synthesizing silicates and high silica aluminosilicates having the CHA framework type require the use of expensive directing agents and/or reaction mixtures with a low water content and hence high viscosity. Both of these features can provide further obstacles to process scale-up.
There is, therefore, a need for a process for synthesizing crystalline silicates and high silica aluminosilicates including a CHA framework type molecular sieve in which the process employs a reaction mixture that is substantially free of added hydrofluoric acid. The desired process would use an inexpensive structure directing agent and/or a high water content. The desired process would ideally be applicable to not only silicates and aluminosilicates, but also to the synthesis of CHA framework type molecular sieves in which part of all of the silicon is replaced by another tetravalent element, such as tin, titanium, germanium or a combination thereof, and part or all of any aluminum is replaced by another trivalent element, such as boron, iron, indium, gallium or a combination thereof. There is also a need for a method for making a crystalline product that is not necessarily a pure phase CHA material, but can also include stacking faults and/or include an intergrowth of a CHA material with another framework type molecular sieve, such as an AEI framework type material.
U.S. Pat. No. 7,094,389 describes a crystalline material substantially free of framework phosphorus and comprising a CHA framework type molecular sieve with stacking faults or at least one intergrown phase of a CHA framework type molecular sieve and an AEI framework type molecular sieve, wherein said material, in its calcined, anhydrous form, has a composition involving the molar relationship: (n)X2O3:YO2, wherein X is a trivalent element; Y is a tetravalent element; and n is from 0 to about 0.5. The material exhibits activity and selectivity in the conversion of methanol to lower olefins, especially ethylene and propylene.